This invention relates to bonding and, more particularly, to solder bonding of dense arrays of small-area contact pads.
Solder bonding is a commonly used technique for attaching electronic components to each other. Thus, for example, semiconductor chips can be thereby connected to each other in a standard process usually referred to as flip-chip bonding. Or chips can thereby be connected to a circuit board. In either case, solder bonding is effective to establish electrical contact and mechanical connections between aligned solder-containing contact pads on the respective components to be interconnected.
Conventional solder-bonding processes involve melting and remelting of solder layers or bumps in a liquid or gaseous flux. To limit lateral movement of molten solder during such bonding processes, and thereby to decrease the likelihood of shorts occurring between pads, it is often necessary to form a so-called dam around each solder area. This of course complicates the structure and increases the cost of fabricating components having solder-containing contact pads.
Solder bonding of dense arrays of microminiature contact pads is especially difficult. In typical such arrays, the pads are positioned extremely close together and each pad includes multiple very thin layers. Maintaining the geometry and cleanliness of such pad arrays during melting and remelting of solder is particularly challenging. Further, molten solder may interact with constituents in other layers of the pads to deleteriously change the electrical and/or mechanical properties of the pad structures. Also, when interconnecting components formed on substrates having different coefficients of thermal expansion, raising the temperature of the components during solder melting and remelting may introduce sufficiently harmful stresses to break some or all of the interconnections.
Accordingly, efforts have continued by workers skilled in the art directed at trying to devise effective solder-bonding techniques particularly suited for connecting together dense arrays of microminiature contact pads. It was recognized that such efforts, if successful, could improve the yield and thereby lower the cost of making interconnections between electronic components that contain small-area multi-layer solder-containing contact pads.